tensile properties of steel
TRANSCRIPT
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Tensile Properties of Steel
Mechanics of Materials
Pete Zumpano
2/6/2012
MVCC
Mr. Restive
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Objective:
In order to understand and observe the effects of strain on a material, an experiment
was devised to test the strength of a 0.505 steel bar by putting the bar into tension. The goal
of the experiment was to measure the effects of tension being increased on a steel bar which
creates a strain on that tensile specimen. Fundamental stress formulas were used to model thebehavior of the specimen in tension.
Equipment:
The following was used in the tensile test experiment:
Tinius Olsen Tensile tester Dial caliper Ruler Extensometer/Strain gauge Anvil puncher Hammer
Tinius Olsen Tensile Tester, smallest deviation 50 lbs
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Force Reading Face of tensile tester
The main machine used to test the tensile strength of the steel specimen was a Tinius Olsen
tensile tester. This machine has threaded attachments for which the steel specimen can be
screwed into. The tensile test is hydraulically driven and the amount of force exerted can be
read on a large dial to the right of the machine.
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Extensometer, smallest deviation .0001
Another important piece of equipment is the extensometer, or strain gauge. The strain gauge
has two arms that are set to attach to the specimen two inches apart by using the anvil puncher
which in imprints two holes, two inches apart.
Specimen to be tested
Anvil puncher
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Dial Caliper
Procedure:
To begin the experiment, first a table was created to hold our data while the experiment
was being conducted. The data table had four columns consisting of Load (lbs), Stress (psi),
Deformation (in), and Strain (in/in), however, for conducting the experiment only the Load and
Deformation column were used. Following, the strain gauge was lined up with the holes from
the anvil puncher and then fastened to the steel specimen. Then, the specimen was attached to
the tensile tester and the tester was zeroed. After, a load of 500 lbs was placed on thespecimen to remove any looseness in the machine and the strain gauge was zeroed. At this
point in the experiment, all of the pre-setup is completed and the experiment can initiate.
Following the initial setup, the tensile tester was increased in 500 lbs increments and
the amount of deformation from the strain gauge was measured. This process was continued
until the yield point of the specimen was reached; this data point was noted. The process of
increasing the force on the specimen by 500 lbs was continued until the ultimate load point was
reached and the strain gauge was removed. Finally, the force on the specimen was increased
more until the breaking point of the steel specimen. The force at the break point was recorded
and the final diameter of the specimen was measured at its smallest point.
Results and Discussion
Once the data from the experiment was taken, the majority of the experiment can be
analyzed using one graph; the stress versus strain graph. This graph illustrates all of the
important points of the specimens mechanics including the proportional limit (elastic limit), the
yield strength, and ultimate strength.
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Beginning from when the specimen was loaded with 500 lbs, the stress vs. strain graph
appeared to be increasing linearly. This occurrence implies that in the beginning of the loading
stress is proportional to strain and is related by some constant. The linear relationship observed
is also apparent in the study ofsprings described by Hookes law where force is proportional to
the distance a spring is stretched. For stress versus strain, the relating constant is known as the
modulus of elasticity (E). In, terms of our experiment, the modulus of elasticity is the slope ofthe stress vs. strain graph which can be calculated by finding the change in stress over the
change in strain from point to point of our calculated values of stress and strain. Then, an
average can be taken of these values to find the modulus of elasticity of the specimen. It is
important to note that only the linear part of the stress versus stain graph can be used to find E.
The average modulus of elasticity was calculated to be 30088795.38 psi. The trend line on the
figure 1 represents the fitted modulus of elasticity, however, for the experiment the average
modulus of elasticity for calculations and comparisons. (Refer to figure-1)
Figure-1, from initial loading to near proportional limit
After the linear portion of the graph, the strain in the specimen begins to change
nonlinearly. The boundary between were the curve transition from linear to nonlinear is called
the proportional limit, or elastic limit. Up until this point, if strain on the specimen was reduced,
the specimen would shrink back to its original size following Hookes Law. The proportional
limit was determined to be at 79,000 psi. However, after the proportional limit, the yield
y = 30,723,036.28x + 2,496.00
R = 0.990.000
10000.000
20000.000
30000.000
40000.000
50000.000
60000.000
70000.000
80000.000
0 0.0005 0.001 0.0015 0.002 0.0025
(
psi)
(in/in)
Stress vs. Stain
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strength is soon to be reached which is the point where plastic deformation begins to occur and
subsequent strain will result in permanent deformation. Yield strength can be estimated using
the 0.2% offset method, (explained in further section) and in this experiment was approximated
to be 94,000 psi (Refer to figure-3)
Figure-2, zoomed in where graph becomes nonlinear
In order to find the yield strength, it is common to use a 0.2% offset method to find the
estimated yield strength. To perform this method, 0.2% is added to the strain from the initial
data point of the stress versus strain graph. Then, a parallel line to the linear portion of thestress versus strain graph is draw until it interests the original graph. The 0.2% offset line
intersects the original stress versus strain graph in the nonlinear part. The point at this
intersection is considered to be the yield strength. The stress at this point is referred to as the
yield strength, for after the yield strength, the material begins to be nonlinearly strain and any
addition stress will cause permanent deformation of the specimen.
0.000
20000.000
40000.000
60000.000
80000.000
100000.000
120000.000
0.001 0.011 0.021 0.031 0.041 0.051 0.061
(
psi)
(in/in)
Stress vs. Stain
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Figure-3, where the 0.2% offset line crosses the stress vs. strain curve is the yield
strength
After the yield stress was passed and when stress was further increased, the material
began to deform to the point where the middle portion of the specimen narrowed. At this time,
the material approached its ultimate strength, or the maximum amount of stress it can
withstand which was recorded at approximately 103594.60 psi. Following the ultimate
strength, the specimen narrowed even more at the middle until it broke in half at the breaking
force of 14,750 lbs at a stress of 73639 psi. Several quantitative tests were performed on the
specimen to view its reaction to the stresses applied to it including the percent area reduction,
percent elongation, and toughness. The percent area reduction was found to be 45.43 % and
the percent elongation was calculated to be 18.45%. These values show how much physicalchange the specimen can withstand before breakage. The toughness is measured by
determining the area under the curve of the stress versus strain graph. Toughness refers to how
much energy a material can absorb without breakage. An estimation of toughness can be
determined using . A more accurate representation of toughness is the areaunder the stress versus strain curve. The toughness of the specimen was calculated to be
18621.19 psi by find the area under the curve.
y = 3E+07x - 53014
R = 0.9976
0.000
10000.00020000.000
30000.000
40000.000
50000.000
60000.000
70000.000
80000.000
90000.000
100000.000
110000.000
120000.000
130000.000
140000.000
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04
(
psi)
(in/in)
Stress vs. Stain 0.2% Offset
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Conclusion:
At the termination of the experiment, the specimen was concluded to be 1018 steelsince the specimens modulus of elasticity was calculated to be 30,088,795.38 psi which
compared well to a known 1018 steels modulus of elasticity of 29,000,000 psi. The
experimental result was just 3.4% off of the actual modulus of elasticity value. Also, both the
percent area reduction and percent elongation were similar to an actual 1018 steel (refer to
appendix). With the three aforementioned figures, there is a strong indication that the
specimen is 1018 steel.
References:
Don, Josen. "Steel-Tube_SAE 1018 Steel Properties." Steel-Tube_Seamless Steel
Tube_Precision Steel Tube_Cold-rolled Steel Tube_Clod-drawing Steel Tube. 23 May
2009. Web. 09 Feb. 2012. .
"Tensile Testing." Instron. Web. 2 Feb. 2012.
.
Appendix:
Definitions:
Stress - is the amount of force per perpendicular amount of area; measured in psi.
Strength - the amount of stress a material can withstand
Yield Strength -the point at which the specimen begins to be permanently deformed
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Ultimate Strength- the maximum amount of load able to be applied to the specimen
Proportional limit- the point of a stress vs. strain curve where the graph becomes nonlinear
Elastic limit- same as proportional limit
Strain- is the amount of deformation of material per original length of that material.
Sample Calculations:
Calculation of Stress for when Force equals 1000 lbs
Calculation of percent elongation:
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Calculation of reduction of area:
Calculation of modulus of elasticity for first two data points of stress and strain:
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Calculation of toughness:
()
Calculation of Toughness from the area under the curve:
Done with excel:
1) Find difference between each strain point
2) Multiply difference (x) by stress at that point
3) Sum all of the stress times x values to gather the total area under the curve
The total area under the curve was found to be 18621.19 psi for toughness.
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